1. Field of the Invention
The present invention relates generally to a visible ray communication system, a transmission apparatus, and a signal transmission method.
2. Description of the Related Art
An optical communication technology using light in a visible ray region has recently been explored. In particular, as an illuminating device using a luminous element, such as an Light-emitting Diode (LED) Light-emitting, has rapidly come into use, development of technology for achieving higher speed, convenient data communication by utilizing an infrastructure, such as an interior/exterior illuminating device, has been carried out.
As a light-emitting device used for high-speed optical data communication, an LED is the most powerful candidate in consideration of an influence on a human body or medical appliances. A semiconductor light-emitting device with a higher speed response performance, such as a Laser Diode (LD) or a Super Luminescent Diode (SLD), is proposed as a candidate. The data transmission speed in the optical communication depends on a response speed of a light-emitting device. Thus, such a light-emitting device with a higher speed response performance is required.
Also, in order to further improve the data transmission speed, a technology for transmitting a large amount of data during one signal emitted from a light-emitting device in a stable manner is required. One example of optical communication technology, Japanese Patent Laid-Open Publication HEI 2008-252444, which discloses a technology of removing spatial interference by allocating a time axis of an Orthogonal Frequency-Division Multiplexing (OFDM) signal in a spatial direction.
When OFDM is used, it is possible to improve the frequency use efficiency and the endurance against multi-path. Thus, the OFDM is widely used in a wireless communication system (e.g., a wireless LAN) or a wired communication system (e.g., Asymmetric Digital Subscriber Line (ADSL)). In visible ray communication, the communication quality is expected to be improved by the application of OFDM. However, in OFDM, there is a problem in that a Peak to Average Power Ratio (PAPR) is increased. In other words, a large dynamic range is required in a transmitter and a receiver.
Accordingly, when OFDM is applied to visible ray communication, a very large current flows in an LED, as a transmission device of the visible ray communication. For example, in an LED, a current ranging from several hundred milliAmps (mA) to several Amps (A) flows. For this reason, a drive circuit capable of covering a signal with a very wide dynamic range in a transmission-side is necessary. However, in general, an LED is used for emitting light with a predetermined quantity, and, thus, is actually not appropriate for handling such a wide dynamic range signal because it requires a special element.